The present invention is related to the determination of temperature and the measurement of temperature profiles in stationary and flowing radiation translucent materials. In particular, the present invention is related to a non-contact means for determining temperatures, temperature profiles and material thicknesses of such materials using transmitted and emitted electromagnetic radiation in a selected wavelength range.
Precise measurement and control of material temperature during processing is essential to ensure consistent physical properties in the final product. Traditionally, thermocouples have been used to obtain an indication of material temperatures. Such measurements, however, are prone to errors due to several factors; frictional heating of the probe in a flowing system, heat transfer to the thermocouple can also contribute to erroneous temperature indications. Also, thermocouples cannot be applied to materials such as thin films as they are an intrusive device. It is thus seen that thermocouples immersed in materials are not wholly satisfactory instruments for determining temperature because of erroneous measurements arising from frictional heating of the thermocouples themselves.
Additionally, such thermocouples generally exhibit a relatively slow response time. Moreover, surface temperature measurements performed on polymer melts using emitted radiation are limited because this measurement method is based on the fact that, at certain wavelengths, polymers behave like opaque materials. While this, phenomena enables one to determine the temperature of a given surface by measuring the radiation intensity emitted from that surface, it none the less does not provide any means for determining interior temperatures which may, in fact, be more critical for controlling ultimate material properties.
Recently, infrared detectors have been employed in a limited fashion to measure polymer surface temperatures. However, such infrared detectors have not been useful in obtaining accurate bulk temperature and temperature profiles. Their use has been confined to surface temperature measurement. See "Infrared Radiation Techniques for Glass Surface and Temperature Distribution Measurements", by R. Viskanta in the IEEE Trans. Indust., Appl. Vol 1A-11, No. 5 (1975).
A method to estimate bulk temperature has recently been described, U.S. Pat. No. 4,516,864. This method has limitations due to the necessity to characterize the polymer temperature absorbance coefficient at the wavelength of the collimated sources. A further difficulty with this invention is its inability to deal with the effect of polymer thickness on emissivity.
Therefore, it is apparent that a method and apparatus be developed that provides accurate non-contact measurement of internal temperature and temperature profile of materials without previous knowledge of their thickness or radiation interaction characteristics.